Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system of an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals. The resulting errors end up revealing Eve's presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). A specific QKD system is described in U.S. Pat. No. 5,307,410 to Bennett (the '410 patent), which patent is incorporated herein by reference.
The Bennett-Brassard article and the '410 patent each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization of single photons, and Bob randomly measures the polarization of the photons. The one-way system described in the '410 patent is based on a two-part optical fiber Mach-Zehnder interferometer. Respective parts of the interferometer are accessible by Alice and Bob so that each can control the phase of the interferometer.
QKD systems include an “optics layer” having one or more optical circuits and an electronics layer having one or more electronic circuits. Proper operation of the QKD system requires precise and repeatable timing, which involves making sure that electrical signals in the electrical circuits are at the correct values when the quantum signal (photon) is at particular point in the optical circuit. FIG. 1A is a schematic diagram of a generalized prior art two-way QKD system 10A, such as disclosed in U.S. Pat. No. 6,438,234, showing a number of elements critical to the timed operation of the system. Those elements not critical to the timing operation of the system have been omitted for the sake of illustration. The optical paths are indicated by dark lines and the electrical paths are indicated by lighter lines.
FIG. 1B is a schematic diagram of a generalized prior art one-way QKD system 10, such as disclosed in U.S. Pat. No. 5,675,648, showing a number of elements critical to the timed operation of the system. Those elements not critical to the timing operation of the system have been omitted for the sake of illustration. The optical paths for single photons are indicated by dark lines and the electrical paths are indicated by lighter lines.
QKD system 10A of FIG. 1A includes a laser source LS optically coupled to a modulator M1 via an optical fiber section F1, and a modulator M2 optically coupled to modulator M1 via an optical fiber section F2. Modulator M1 is optically coupled to two single-photon detectors SPD1 and SPD2 via respective optical fiber sections F3 and F4. Laser source LS is electrically coupled to a timing generator TG that controls the timing of the emission of a light pulse LP. A clock CLK is electrically coupled to timing generator TG to provide a clock signal for system timing. System 10 also includes two discriminators D1 and D2, where discriminator D1 is coupled to detector SPD1 and to timing generator TG, and discriminator D2 is coupled to detector SPD2 and to the timing generator.
In the operation of system 10A, clock CLK provides a clock signal S1 to timing generator TG to serve as a timing reference. Timing generator TG then generates a timed signal S2 to laser source LS to initiate the activation of laser source LS to emit light pulse LP. Timing generator TG also generates a first modulator signal S3 timed to activate modulator M1 when light pulse LP is passing therethrough. Likewise, timing generator generates a second modulator signal S4 timed to activate modulator M2 when light pulse LP is passing therethrough.
In addition, timing generator TG generates detector signals S5 and S6 to SPD1 and SPD2, respectively, and corresponding signals S7 and S8 to discriminators D1 and D2, respectively. The detector signals S5 and S6 are timed to activate detectors SPD1 and SPD2 at the expected arrival time of light pulse LP at one of the detectors (the arrival of light pulse LP at one of the detectors depends on the modulation imparted to the light pulse LP). In response to a detection event (“click”), SPD1 and SPD2 generate respective SPD signals S9 and S10 that are sent to discriminators D1 and D2, respectively. The discriminator signals S7 and S8 are timed to correspond to SPD signals S9 and S10 to discriminate against detection events other than those caused by the expected arrival of modulated light pulse LP.
The operation of system 10B of FIG. 1B is similar to the operation of system 10A, with two single-photon detectors SPD1 and SPD2 optically coupled to modulator M2.
In systems 10A and 10B, there is a finite time delay between the elements that varies due to, for example, the length of the optical path connecting the various optical elements, and the length of the electrical path (e.g., wires) connecting the various electrical elements.
Timing generator TG is an electrical circuit that includes an inherent amount of timing error. Timing generators that have the ability to generate the type of signals needed—whether they be a microprocessor, a digital signal processor (DSP), a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), or other type of circuit—typically do not have as low a jitter as desired due to the type of logic families used, mostly CMOS and NMOS. While the best of these (FPGAs and ASICs) are good enough to use for classical telecommunications timing applications, they are not good enough for practical (e.g., commercial or non-laboratory) QKD systems. In practical QKD systems, variations in timing that would otherwise be acceptable in a classical telecommunications application are not acceptable in a QKD system because they result in unacceptably high error rates and potential security breaches.
Accordingly, there is a need for timing systems and methods for QKD systems that provide stricter limits on timing variations (e.g., jitter) than are presently used for classical telecommunication applications.